The M-2, nicknamed "Ma Deuce" by the troops, has been around so long because it was very good at what it did. Accurate, reliable, rugged and easy to use, many of the M-2s currently in use are decades old, and finally wearing out. The army doesn't want to build new ones, and wasn't sure it could do without the venerable, and very useful, Ma Deuce. So it ended up going ahead with the plan to build a new .50 caliber machine-gun (the XM312). Actually, the new "Ma Deuce" is basically the XM307, but without the ability to fire 25mm rounds. The XM312 weighs 36 pounds (compared to 50 for the M-2), even with the addition of the electronic fire control stuff from the XM307.
The fire control system, especially the range finder, makes the XM312 much more accurate with first shot hits, than the M-2. American troops have been testing the XM312 in the United States and Germany, and have also reacted favorably to the lighter weight of the XM-312. The lighter XM312 will be easier for infantry to manhandle into position (along with its tripod mount.)
The XM-312, the replacement for the eighty year old, .50 caliber (12.7mm) M-2 ("Ma Deuce") machine-gun, is now undergoing field testing, in the United States and overseas. Originally, the replacement was going to be the M-307, which was designed so it could fire either the computer controlled 25mm "smart shell" of the XM-25, or (by changing the barrel and receiver), .50 caliber ammo. But it was felt that a straight replacement for the M-2 was needed quickly. The troops will begin getting the XM312 in 2008, or sooner.